Acidic Site-Controlled ZSM-5 Catalysts for Fast Molten-Phase Pyrolysis of Plastic Waste with Tunable Product Distribution

The catalytic pyrolysis of plastic waste into fuels has attracted considerable attention due to its potential to address the energy crisis and alleviate public health concerns. However, current catalytic pyrolysis systems for waste plastics still face challenges, such as low efficiency, poor product selectivity, and a lack of monitoring during the pyrolysis process. In this study, highly stable ZSM-5 zeolite catalysts were synthesized with tunable Si/Al ratios for the efficient cracking of high-density polyethylene (HDPE) into oil- and hydrocarbon-rich gases. It was found that a decrease in the Si/Al ratio of the ZSM-5 catalyst enhanced its acidity and cracking efficiency for HDPE, as confirmed through a one-pot melting-catalysis strategy and thermogravimetric analysis. The increased acidity of the ZSM-5 catalyst not only improved the cracking efficiency but also facilitated the regulation of product distribution, offering valuable insights into the design of highly selective catalysts. ZSM-5 (22) exhibited the highest conversion rate, achieving nearly 100% HDPE conversion at 450 °C in 19 min and full conversion at a lower temperature of 350 °C over an extended period, and demonstrated good stability and recyclability. The acidic properties and the aluminum coordination state of the prepared ZSM-5 catalysts were examined with a proposed mechanism for high-density polyethylene (HDPE) cracking based on the acidic sites of ZSM-5. Furthermore, the catalyst demonstrated its efficacy in the pyrolysis of actual plastic waste. These findings highlight the potential of ZSM-5 catalysts for the cracking of plastic waste through the one-pot melting-catalysis strategy, offering valuable insights to further advance dynamic studies of plastic cracking processes.